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The Boundary Diagram

Better System and FMEA Design with Block Diagrams

The FMEA steps such as defining the scope, developing system structures and functional analyses are more likely to succeed with boundary diagrams. Boundary diagrams are part of the tools of every experienced FMEA moderator.

Task formulation

Developing one or more boundary diagrams together with the responsible experts, before starting the system and FMEA design and as preparation for the respective object of analysis. This diagram form has proven to be the ideal tool for preparing FMEAs.


The Boundary Diagram is used to define the system to be analysed. This is done by making the necessary decisions about the location of the external interfaces. Those who define interfaces by and large define the system. The system definition also includes the determination of the inherent subsystems and their internal interfaces. With these, a further objective is also realised: Preparing the functional analysis. The interfaces represent the locations of the functions.


1. Give the system a name. For instance: Cordless mouse.
2. Define the associated development and/or delivery scope. For instance: Cordless mouse, USB plug, battery.
3. Display and delimit corresponding but not associated systems. For instance: Notebook.
4. Identify and document the external interfaces. For instance, the interfaces.
      a. Cordless mouse/computer system
      b. Cordless mouse/hand
      c. Cordless mouse/table surface
      d. Cordless mouse/setting

5. Create a table of the external interfaces with the corresponding functions.

6. Represent the inherent systems in the boundary diagram. For instance: Battery, circuit board, etc.
7. Identify and document the interfaces of the inherent systems. For instance, the interfaces
     a. Printed circuit board / housing
     b. etc.
8. Create a table of the internal interfaces with the corresponding functions.


This information can now be used for the following steps in FMEA development: Define Application Area, Develop System Structure, and Perform Functional Analysis.


The result indicates many discerning discussions of the FMEA team regarding application area, system structure and functional analysis which can be moderated safely and are at disposal of FMEA development as input variables.

It is also possible to use the interfaces themselves as system elements in the FMEA system structure. Thus a functional analysis can be realised, which can be well developed.

The visual representation of the development and delivery scope can be provided, in case they differ from each other.

Find out more at the 14th FMEA Forum in Osnabrück, 03/27 & 03/28/2019.